Reinitiation at the lambda DNA origin accompanies the host SOS response

Abnormal reinitiation of replication from lambda origins has previously been found during infection in the presence of caffeine or cis-diamminedichloroplatinum II (cis-Pt) or when lambda infects a P2 lysogen. It was further shown that the reinitiations arising from cis-Pt treatment took place during the SOS response induced by the template damage caused by the drug. It is now shown that SOS induction by uv irradiation of the host also results in reinitiation events and that it is the SOS response itself rather than some other direct effect of the damaged host template that is responsible for the phenomenon. Parental sections of lambda replicative intermediates can supercoil, whereas daughter segments cannot. To explain the control that prevents reinitiation, it is proposed that normally the origin sequence has to be under superhelical tension to be a suitable substrate for the initiating machinery; once a round is in progress, the daughter origin sequences would not be under such tension and would therefore be inactive. It is shown that in an SOS environment the proposed requirement for a superhelical origin sequence is relaxed and consequently the control against reinitiation lost. Under such conditions, primary growing points that have encountered template lesions terminate and a new wave of replication initiates.